A measure of the droplet size distribution and thus of the droplet spectrum of the spray jet is, for example, the so-called span value, as is described inter alia in Mescher et al., Gravity affected break-up of laminar threads at low gas-relative-velocities, Chem. Eng. Sci., Volume 69, Issue 1, 13 Feb. 2012, pages 181-192.
The slower the bell disc is rotated, the larger, on average, are the droplets which are hurled away from the breakaway edge.
Accordingly, at higher rotational speeds of the bell disc, on average smaller droplets are produced at the breakaway edge of the bell disc. For this reason, the bell disc is generally operated at high rotational speeds, which involves a correspondingly high energy consumption. At the same time, the radial spreading of the spray jet is, in turn, greater at higher rotational speeds of the bell disc than at lower rotational speeds, so that measures have to be taken to focus this spray jet onto the objects to be coated.
For this purpose, known rotary atomizers operate, for example, electrostatically. In this case, the coating material to be applied is charged, whereas the object to be coated is earthed. During this, an electrical field is formed between the rotary atomizer and the object, by which the charged coating material is applied to the object in a directed manner. However, this works only with electrically conductive objects.
Alternatively or additionally to the electrostatic operation, directing air devices have become established in known rotary atomizers. With these devices, a mostly annular directing air stream is guided onto the spray jet such that the latter is focused and the droplets of different size are guided in a directed manner onto the object to be coated.
For this purpose, however, in some cases strong directing air streams are necessary, the generation of which with known means is relatively costly.